Kingdom Protista for AP Biology
Practice problems for these concepts can be found at: Taxonomy and Classification Review Questions for AP Biology
The endosymbiotic theory states that eukaryotic cells originated from a symbiotic partnership of prokaryotic cells. This theory focuses on the origin of mitochondria and chloroplasts from aerobic heterotrophic and photosynthetic prokaryotes, respectively.
I can see why scientists examining these two organelles would think that they may have originated from prokaryotes. They share many characteristics: (1) they are the same size as eubacteria, (2) they also reproduce in the same way as prokaryotes (binary fission), and (3), if their ribosomes are sliced open and studied, they are found to more closely resemble those of a prokaryote than those of a eukaryote. They are prokaryotic groupies living in a eukaryotic world.
The eukaryotic organism that scientists believe most closely resembles prokaryotes is the archezoa, which does not have mitochondria. One phylum grouped with the archezoa is the diplomonads. A good example of a diplomonad you should remember is Giardia—an infectious agent you would do well to avoid. Giardia is a parasitic organism that takes hold in your intestines and essentially denies your body the ability to absorb any fat. This infection makes for very uncomfortable and unpleasant GI (gastrointestinal) issues and usually results from the ingestion of contaminated water.
The evolution of protists from prokaryotes gave rise to the characteristics that make eukaryotes different from their prokaryotic predecessors. Protists were around a long time before fungi, plants, or animals graced our planet with their presence. Most protists use aerobic metabolism. Since this is a chapter on classification, it would be silly, if not too kind of me, to not mention how these different protists are organized. They are usually grouped into three major categories:
- Plantlike protists: photosynthetic protists, also called algae
- Animal-like protists: heterotrophic protists, also called protozoa
- Funguslike protists: protists that resemble fungi; also called absorptive protists
Protists are usually unicellular or colonial. This is why they are not considered plants, animals, or fungi. All protists are capable of asexual reproduction. Some reproduce only asexually, and others can reproduce sexually as well. This variability in the life cycles found among various members of the protist kingdom is just one reason why they are considered to be one of the most diverse kingdoms in existence.
Animal-Like Protists (Protozoa)
This division includes protists that ingest foods—as do animals. As with the rest of this chapter, you do not need to become an expert on protozoans and know everything about every member. But the following is a list that contains basic information about some names that may help you on the multiple-choice section of the test. I will italicize the most important things to remember about each of them.
- Rhizopoda. These unicellular and asexual organisms are also known as amoebas. They get from place to place through the use of pseudopods, which are extensions from their cells. Every living creature has to eat, and they do so through phagocytosis.
- Foraminifera. These marine protists live attached to structures such as rocks and algae. Their name is derived from the word foramen because of the presence of calcium carbonate (CaCO3) shells full of holes. Some of these protists obtain nutrients through photosynthesis performed by symbiotic algae living in their shells.
- Actinopoda. These organisms move by pseudopodia and make up part of plankton, the organisms that drift near the surface of bodies of water. The two divisions of actinopoda include heliozoans and radiozoans. Just recognize the names; do not worry about anything more than that.
- Apicomplexa. These parasites are the protists formerly known as sporozoans. They spread from place to place in a small infectious form known as a sporozoite. They have both sexual and asexual stages, and their life cycle requires two different host species for completion. An example of an apicomplexa is plasmodium, the causative agent of malaria (two hosts—mosquitoes, then humans).
- Zooflagellates. These heterotrophic protists are known for their flagella, which they use to move around. Like rhizopoda, they eat by phagocytosis and can range from being parasitic to their hosts to living mutualistically with them. A member of this group is trypanosoma, which is known to cause African sleeping sickness.
- Ciliophora. Their name is fitting because these protists use cilia to travel from place to place. They live in water and contain two types of nuclei: a macronucleus (which controls everyday activities) and many micronuclei (a function in conjugation). A ciliaphora you may recognize is paramecium.
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